1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images formed by electrophotography, electrostatic recording method, or electrostatic printing method, to visible toner images.
2. Discussion of Background
Various image recording methods by electrophotography or the like are described, for instance, in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 49-23910 and No. 43-24748. Generally in such recording methods, latent electrostatic images are formed on a photoconductor comprising a photoconductive material and then developed with a toner to visible toner images. When necessary, the developed toner images are transferred to a transfer sheet made of, for example, paper, and fixed thereon by the application thereto of heat and/or pressure, or a vapor of a solvent, whereby image-bearing copies are made.
The methods of developing such latent electrostatic images to visible toner images can be roughly classified into two methods.
One method is a liquid development method which uses a liquid developer comprising various kinds of finely-divided pigments or dyes dispersed in an insulating organic solvent.
The other method is a dry development method which uses a dry toner comprising a coloring agent, such as carbon black, dispersed in a natural or synthetic resin. Specific examples of the dry development method include cascade development, magnetic brush development and powder cloud development.
For fixing toner images on the transfer sheet, a heat roller image fixing method is in general use because of its excellent energy efficiency.
Image fixing by the heat roller image fixing method is conventionally performed by applying heat to toner images at a temperature of 150.degree. to 200.degree. C. for about 0.01 to 0.03 seconds. However, because of the necessity for the reduction of the required thermal energy for image fixing to low energy and for high speed copying, there is a keen demand for a toner which is capable of performing low temperature and quick image fixing.
The trend of reducing the thermal energy for image fixing will further continue.
Generally, toners for such low-energy image fixing have been improved by replacing a resin component in conventional toner with a resin having a low softening point or wax. However, such a low-temperature-fixing toner, however, has the shortcoming that the thermal preservability thereof is so poor that the toner is aggregated outside an image fixing portion by the heat applied thereto which is built up in the mechanical parts of a development unit in contact with the toner, or by some heat applied during the storage of the toner. Such aggregation of the toner is referred to as "blocking phenomenon".
In order to solve this problem, it has been tried to use, as a binder resin for toner, polyester resin which is considered to be suitable for the low-temperature image fixing and to have relatively good thermal preservability. However, there has not been available a toner which has satisfactory low-temperature image fixing performance and thermal preservability for use in practice at the same time. Polyester resin has the shortcoming that when polyester resin is used in the toner, the productivity thereof is considerably reduced in the course of a pulverizing process in the production of the toner because of the high strength of the polyester resin itself.
Japanese Laid-Open Patent Applications Nos. 50-99740, 50-99741, 50-99742, 53-118050 and 54-48556 describe that toners having both the low-temperature image fixing performance, for instance, with an image fixing temperature of 100.degree. to 140.degree. C., and the thermal preservability, can be prepared by use of a petroleum resin as a resin component for the toners.
However, in view of the recent demand for a toner with a further lower temperature image fixing temperature, there has not yet been developed a toner which completely satisfies such demand by mere use of any of conventional petroleum resins as a resin component for the toner.